518 Grout — Lopolith; An Igneous Form 



name. Such a name is better based on the known facts of 

 form or relations than on any theory of origin, and the 

 name proposed by the writer is " lopolith " (from Xo7ra?, 

 a basin, a flat earthen dish, and Xt'flo?, a stone).* 



The lopolith. — A lopolith may be defined as a large, 

 lenticular, centrally sunken, generally concordant, intru- 

 sive mass, with its thickness approximately one-tenth 

 to one-twentieth of its width or diameter. Most of the 

 known lopoliths are in part of basic rocks, and probably 

 because of their large size and slow cooling have 

 differentiated notably. They may show the varying 

 degrees of complexity described as " multiple, " "com- 

 posite, " " divided, ' ' " interf ormational, ' ' as distinguished 

 from "simple." The type departs from a laccolith, not 

 only in form but in the probable mechanics of its 

 intrusion. 



The Duluth gabbro with its differentiates is one of the 

 best illustrations of a lopolith. At Duluth the roof and 

 floor dip east. The crescentic outcrop, concave toward 

 Lake Superior (see fig. 1), dips in all parts toward the 

 lake. The assumed eastern border of the lopolith is con- 

 cealed under other rocks and under the lake, but the 

 sheet of gabbro on the Gogebic range dips north even 

 more steeply than the Minnesota mass dips south. It is 

 thus somewhat unsymmetrical, but clearly sunken in the 

 center. Its cross-section is also clearly lenticular. The 

 overlying rocks are mostly Keweenawan flows, and 

 though the horizon of the roof may vary some hundreds 

 of feet, the discordance is unimportant when compared 

 to a lateral extent of about 150 miles. The base of the 

 gabbro rests on such a series of formations from Archean 

 to Keweenawan, that the first impression is one of com- 

 plete discordance with earlier structure. However, if 

 the intrusion transgressed the earlier structure, it is a 

 remarkable coincidence that the two ends, now outcrop- 

 ping 140 miles apart, and the southern outcrops almost as 

 far to the south, all transgressed up to exactly the same 

 horizon. This coincidence is not the only difficulty in the 

 assumption of a transgressing intrusion. After the bor- 

 ders had transgressed to the Keweenawan, the central 

 parts of the intrusion which must have been in the 

 Archean, must have stoped their way up to exactly the 

 horizon to which the border was first intruded; we now 



* Pronunciation, lo'polith. 



