<&tmxai Notes. 

 petrography; 



Ancient Volcanics in Michigan.— In an area in Michigan 

 covered by Townships 42 to 47 N. and Ranges 30 to 34 West, is a 

 succession of granites and gneisses overlain by a thickness of KtaM 3000 

 feet of volcanic rocks, embracing acid and basic flows and tuffs. 

 Among the basic rocks Clements 2 finds porphyrites and melaphyres, 

 and among the acid ones quartz-porphyries and devitrified rhyolites. 

 The melaphyres and porphyries are described under the names apo- 

 basalts and apo-andesites, because they are altered forms of basalts and 

 andesites. Some of the andesites are amygdaloidal, and nearly all 

 show the effects of pressure. Andesitic and basaltic tuffs are both pres- 

 ent. They exhibit no special peculiarities. The quartz porphyries 

 among the acid flows are notable for the existence in them of corroded 

 phenocrysts of quartz in which there has been developed a well marked 

 rhombohedral cleavage. The groundmass of these rocks is sometimes 

 micro-granitic and at other times is micro-poicilitic. The latter struct- 

 ure is peculiar in that it is produced by a reticulating uet work of uni- 

 formly oriented quartz, between the meshes of which are irregularly 

 shaped areas of orthoclase. The other acid lavas and the acid tuffs are 

 similar to corresponding rocks elsewhere. The series is interesting as 

 affording another illustration of a typical volcanic series of Pre-Cam- 

 brian age. It is one of the oldest accumulations of volcanic debris and 

 lavas thus far described. 



Gneisses of Essex Co., N. Y.— In a recent bulletin on the 

 geology of Moriah and Westport Townships. Essex Co., N. Y., Kemp s 

 gives a general account of the petrography of the gneisses, limestones, 

 black schists, gabbros, anorthosites and dyke rocks of these regions. 

 Most of these rocks have already been described in more detail in 

 other papers. The gneisses are of several varieties. The most com- 

 mon is a member of the basement complex underlying the other rocks 

 of the district. It is a biotite gneiss composed of quartz, micro-perthite, 

 orthoclase, plagioclase and brown biotite, all of which minerals exhibit 

 evidences of dynamic metamorphism. Near iron ore bodies the gneiss 

 becomes more basic, abundant green or black hornblende, green 



1 Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 



» Journal of Geology, Vol. Ill, p. 801. 



8 Bull. N. Y. State Mu< . Vol. 3, No 14, 1S95, p. 325. 



