668 STRATIGRAPHICAL GEOLOGY. 



of Triassic age in Massachusetts, and trachytes. The latter are com- 

 monly regarded as of volcanic origin, or later than Cretaceous. The 

 question of the age of these eruptive rocks is now being studied by Mr. 

 Hawes, and he will be able to present more satisfactory conclusions re- 

 specting them in his chapter on Lithology. 



Equivalen'cv of New IIampsiiire For:matioxs. 



Having stated the probable succession of our formations, it is proper 

 next to correlate them with similar groups in adjacent territory. We 

 find in Canada the systems Laurentian, Labrador, Huronian, and Cam- 

 brian. The first two of our groups may be referred to the oldest of 

 these, the Laurentian, without great hesitation. We do not possess ex- 

 haustive information about the occurrence of this oldest system in other 

 regions. So far as is understood, there are two sorts of associated rocks 

 in its typical localities, one being largely pyroxenic. That variety is 

 wanting in New Hampshire. A porphyritic or angcn gneiss is eminently 

 characteristic of the fundamental rocks in every part of the world, and 

 hence ours may readily be called Laurentian. The Bethlehem group, 

 consisting largely of protogene gneisses, is closely related to Laurentian 

 rocks, though not so characteristic of them as the preceding. 



Those who are familiar with the crystallines, as Prof. Dana (p. 109) and 

 Dr. Sterry Hunt, after examining some parts of the Bethlehem group in 

 New Hampshire, say that there is a close resemblance between them and 

 the Laurentian. In a geological map of New Hampshire and Vermont, 

 published in 1877 for a topographical atlas of the first named state, I 

 have grouped these rocks, the porphyritic and Bethlehem gneiss, as 

 Laurentian. 



The next division, the Lake gneiss, cannot be so readily assigned. Its 

 affinities are strongly with the Laurentian, but it is not pyroxenic nor 

 poriDhyritic, nor does it abound in any triclinic feldspar. The Manchester 

 range is thoroughly crystalline, and shows intensely twisted stratification, 

 features that do not prevail in the other areas. I sometimes think this 

 particular area, as well as its analogue in Berlin, might be separated from 

 the others and put into an older group. In Massachusetts this group 

 carries the Eosoon, but that fossil is not confined to the Laurentian. 

 Limestone is not usually present in this group in New Hampshire. 



