60 NEW YORK STATE MUSEUM 



specially in the Keene valley, two or three distinct sets can be 

 recognized. Similar phenomena on a small scale can be recog- 

 nized in the present lakes. 



The open valley south of Lake Placid gives much evidence of 

 having once been a lake whose waters were held in, perhaps by an 

 ice wall to the north. As earlier stated the stumps of the sandy 

 plain, and the deltas need to be correlated as regards altitudes 

 before we can be sure of the conditions surrounding the former 

 lake. The valley in which the town of Wilmington lies is a strik- 

 ing case of a lake basin, and not less significant are the lake bot- 

 toms and deltas in the Keene valley and in the Elizabethtown 

 valley. The latter is almost diagrammatic. 



Geologic age. The hard crystalline rocks are of pre-Cam- 

 brian age, with the possible exception of the trap dikes. If the 

 word archean is used in the original sense as proposed by the late 

 Prof. Dana, for the formations that precede the fossiliferous 

 strata, then the Lake Placid crystallines are archean. But if, 

 as has been more recently proposed by the United States geologi- 

 cal survey, the name archean is resti'icted to those ancient rocks 

 that antedate all sediments, then the local formations must be 

 called Algonkian, a name that applies to pre-Cambrian rocks that 

 are sedimentary, or, if igneous, that are later than known sedi- 

 ments. 



The name Laurentian has been widely employed for the ancient 

 crystalline rocks in the text-books on geology, and as it was origi- 

 nally used in Canada for rocks geographically and geologically 

 related to those under consideration here, it may be referred to. 

 The Canadian geologists introduced the name Laurentian for the 

 oldest crystalline rocks of the globe, and set off from them under 

 the name Huronian, the metamorphosed sediments and igneous 

 rocks that rest upon the Laurentian around Lakes Superior and 

 Huron. With the exception of the trap dikes, the Lake Placid 

 rocks are all Laurentian, but no Huronian rocks are known in the 

 region. 



Fairly extended observation throughout the Adirondacks has 

 led to the conclusion that the limestones, quartzites and probably 



