John McPhee is the world's best translator of shoptalk (I am speaking loosely , of course). In his early books (''Oranges'' is my favorite) he did an extraordinary job persuading us that with him at our elbow all trade secrets were open to us.
But when he chose to write about geology in ''Basin and Range'' (1981) he apparently decided that the time had come to give it to us straight. He and his publishers must have been satisfied with the result, for here he is again with In Suspect Terrain (New York: Farrar, Straus & Giroux, $12.95), still writing about geology, still making few concessions to those of us who don't know an ice-age kettle from a cooking pot.
In fact, I think he is daring us. Okay, he seems to be saying, you say you care about the extraordinary, dramatic way your world was built billions of years ago, so prove it - if you can. And off he goes refusing to talk down to us , insisting that we stick with him through early pages full of words like ''lobate'' and ''menaccanite.''
(He doesn't even explain what ''conodonts'' are until ten pages after he first mentions them. Actually they're ''hard fragments of the bodies of unknown marine creatures - hard as human teeth, and of the same material.'' Very important they are when it comes to identifying rocks and where they originated).
We must let him teach us about the differences that have sprung up between geologists over the importance of plate tectonics, and explain why his companion , geologist Anita Harris, who first espoused and even helped to promote the theory, now thinks it is too glibly applied. Then when he has decided we are worthy of hearing all this (and we have decided it is worth listening to), we begin to appreciate the marvelous dizzying conceptions that roll off his pen. And his humor too. Listen to this, for instance:
''Human time, regarded in the perspective of geologic time, is much too thin to be discerned - the mark invisible at the end of a ruler. If geologic time could somehow be seen in the perspective of human time, on the other hand, sea level would be rising and falling hundreds of feet, ice would come pouring over continents and as quickly go away. Yucatans and Floridas would be under the sun one moment and underwater the next, oceans would swing open like doors, mountains would grow like clouds and come down like melting sherbet, continents would crawl like amoebae, rivers would arrive and disappear like rainstreaks down an umbrella, lakes would go away like puddles after rain, and volcanoes would light the earth as if it were a garden full of fireflies.
''At the end of the program, man shows up - his ticket in his hand. Almost at once, he conceives of private property, dimension stone, and life insurance.''
So he doesn't always need a many-syllabled word to convey the full glory and poetry of the movement of the rocks. Or to give us the paradoxical feeling that we are not only as insignificant as a pinprick but immeasurably momentous.
Now that we have passed our test we can relax and begin to be fascinated with the way Mr. McPhee salts his account with tantalizing snippets of information.
He tells us about painters (the Hudson River School) and explains that the US has enough coal ''to keep Ireland warm for a thousand years.''
I like the boast of the university professor in Bloomington, Ind.: ''You people in New York may have your Empire State Building. But out here we have the hole in the ground it came from.'' (City skyscrapers have a lot to say about rocks.)
There is something appealing about the idea of diamonds that, under more suitable conditions, would have taken form as graphite instead (''these finger-flashing symbols of the eternity of vows, yearning to become fresh pencil lead'').
And, just to make doubly sure that we don't take his tremendous subject too solemnly, John McPhee offers this pleasing notion: ''While geologists argue, the rocks just sit there. And sometimes they smile.''