While the igneous rocks of the Boston basin have been the subject of considerable investigation and discussion, the volcanics of the limited portion of the basin drained by the Neponset river have received little attention. No petrographic study has been made of these volcanics.

Recently Professor W. O. Crosby afforded the writer an opportunity to visit this district and to collect specimens, and further encouraged a more detailed study of the material collected.

The general geology of the region has been investigated by Professor Crosby and is quite aside from the purpose of this paper. A brief sketch of the results of the investigation,* so far as they have appeared, may not, however, be out of place.

Structure of the Region

The prevailing rock is a conglomerate and the prevailing structure is anticlinal. Along the northern margin of the belt the conglomerate dips to the north under a slate, . . .

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