GEOLOGY OF THE SCHROON LAKE QUADRANGLE 93 



SUMMARY OF GEOLOGICAL HISTORY^ 

 Prepaleozoic History 



The Grenville series comprise the oldest known rocks of the 

 quadrangle. They are metamorphosed sediments usually with 

 their stratification more or less well preserved. They are thought 

 to be of Archeozoic age ; that is, they belong among the oldest 

 known rocks of the earth. Since Grenville strata are widespread 

 in northern New York and much of eastern Ontario, it is probable 

 that they were deposited under sea water, and, since they are at 

 least some miles thick, the time required for their deposition must 

 have been no less than a few millions of years. In the Adirondacks 

 the Grenville strata were probably subjected to static metamor- 

 phism whereby the original shales, sandstones and limestones were 

 completely crystallized into various gneisses and schists, quartzite 

 and crystalline limestone (marble). The Grenville strata show a 

 very irregular or " patchy " distribution because they have been 

 badly cut to pieces by great bodies of intrusive rocks. Because of 

 the breaking up of the strata into masses great and small by the 

 intrusives, they are in many places highly tilted or moderately 

 bent, and in some places locally contorted, but there is no evidence 

 that they were ever notably folded by erogenic pressure. In fact 

 the combination of well-preserved stratification of the thoroughly 

 crystallized sediments and foliation due to flattening of minerals 

 always parallel to the stratification would scarcely be expected if 

 severe lateral compression of the strata had ever taken place. 



The oldest known intrusive is the anorthosite which worked its 

 way up into the crust of the earth as a relatively stiff gabbroid 

 magma, probably laccolithically. From the gabbroid magma the 

 anorthosite differentiated. The rising, magma for most part lifted 

 or domed the Grenville strata over it, but also to some extent its 

 borders engulfed fragments of the strata. 



Distinctly later, though apparently not very much (geologically) 

 later, came the intrusion of a tremendous body of magma now 

 represented by the syenite-granite series. This vast magma for 

 most part slowly and very irregularly worked its way upward, and 



^ The interested reader who may not be very familiar with the science 

 of geology might care to consult a recent work by the writer entitled 

 The Adirondack Mountains, which is a somewhat untechnical guide to the 

 geology and physiographic history of the Adirondack region. This is 

 published as New York State Museum Bulletin 193. 



