Fkbudary 16, 1883.] 



SCIENCE. 



39 



pressure of its contents, upon the isolated arterial 

 bulb. The most striking temperature observation is, 

 that the bulb, when brought to heat-standstill at or a 

 little above 40° C, will nearly always beat again if 

 the temperature be still raised two or tliree degrees. 

 H. Nevvei.l Martin. 



LETTERS TO THE EDITOR. 

 Algae and .spray markings. 



Incidental to a note in Nature (xxvii. 46) on 

 Invertebrate casts versus algae in paleozoic strata, 

 the writer would call attention to the fact, that he lias 

 seen many track-like markings made by dried sea- 

 weeds blown along the shore. In some cases a series 

 of parallel indentations, as if some animal had walked 

 along, were made by the stiff projections of the roll- 

 ing plant. These algae tracks and markings are very 

 similar to many fossil tracks whicli have been figured. 



Forms similar if not identical with tliose described 

 by Billings as Arenicolites spiralis from St. .John's, 

 Newfoundland, liave been seen by tlie writer to be 

 formed on the beacli by the spray. This was especially 

 observed last autumn at Marblefiead Neck. The spray 

 dashing over a projecting rock, and falling on the 

 wet sands left by the retreating tide, produced a series 

 of drop and ring like markings in the sand, varying 

 in size from minute drops to those one or two inches 

 in diameter. This corresponds, as regards size, with 

 the specimens of Arenicolites collected by the writer 

 at the Newfoundland locality. Tlie common form of 

 the larger spray markings is that of a ring, with a 

 raised centre and a depressed border, suri-ounded by 

 the displaced sand. The appearance is as if the drop 

 fell like a partly closed bell of a jelly-fish, and then 

 expanded outward in every direction, carrying the 

 sand with it, but leaving the central portion un- 

 touched. These forms would probably be somewliat 

 modified by the next tide, causing variations in the 

 structure, if not obliterating the forms for the most 

 part. As in Newfoundland, so on this modern beach, 

 the impressions are seen crowded together, as well as 

 singly. (See Can. nat., (2), vi. 478 ; Geol. survey 

 Canada, pal. f OSS., ii.77; Amer. journ. .sc, (.3), iii. 223.) 

 M. E. Wadswobth. 



Cambridge, Mass., Jan. 9, 1883. 



Geology of Lake Superior. 



I am pleased to learn from a communication pub- 

 lished in your number of Feb. 9, and signed A. R. 

 C. Selwyn, that the present head of the Geological 

 survey of Canada has arrived at conclusions with 

 regard to the geology of the Lake-Superior region 

 precisely similar to those reached and published by 

 Foster and Whitney over thirty years ago. 



That it would have been well for the Canada sur- 

 vey, and for geological science generally, if more 

 attention had been paid by Jlr. Logan and his assist- 

 ants to the results of the survey carried on along tlie 

 south shore of the lake by the U.S. geologists, during 

 the years 1S4S to 1850, will, it is thought, become 

 apparent to every geologist who reads a work pre- 

 pared by Dr. Wadsworth and myself, soon to appear 

 in the bulletin of the Museum of comparative zool- 

 ogy, and entitled ' The azoic system and its subdi- 

 visions.' J. D. Whitney. 



Cambridge, Feb. 12, 1883. 



Rock disintegration in hot, moist climates. 



Some remarks of Nordenskiold, in his ' Voyage of 

 the Vega,' pp. 707-713, relating to precious stones, 

 suggest the thought that the marked differences 

 which occur as to the manner and rate of the weath- 

 ering of granitic rocks at the north and at the south 



can hardly be so familiar to European scientific men 

 as tliey are to American observers. At the south it 

 is common enough to find soils that have been formed 

 ' in place,' from the thorough and deep-seated chemi- 

 cal decomposition of the rocks on wliich they rest ; 

 while at the north, well-marked disintegi'ation of this 

 sort is rarely met with, even in places wliere the ob- 

 server is not perplexed and confused by the mecliani- 

 cal results of glacial action. The subject lias often 

 been alluded to by American geologists, working in 

 our southern states, notably by Professors Kerr of 

 North Carolina, and Stubbs of Alabama, who have 

 expressed themselves in the following terms: Speak- 

 ing of the geologic formation which, "after hugging 

 the east side of the Appalachian chain of mountains 

 and forming some of the most valuable farming 

 lands of the Atlantic states, entei-s the central eastern 

 part of Alabama," Professor Stubbs says, "The rocks 

 which by disintegration have given the soils of this 

 section are mainly granites, gneisses, feldspars, horn- 

 blendes, mica-schists, etc. ; and much the greater 

 part of the section is covered by soils which have 

 resulted from disintegration of the above-mentioned 

 rocks in situ. And here I may remark a notable 

 feature of these soils, — a feature which cannot fail 

 to arrest the attention of every nortliern geologist: 

 viz., that decomposition of these rocks in southern 

 latitudes has iJroceeded much farther than witli tlie 

 same rocks in liigher latitudes, and therefore has 

 given us deeper soils. It is difficult to find in the 

 north a soil over a few feet deeiJ ; while here it is not 

 uncommon to find in railroad-cuts, wells, etc., disin- 

 tegrated strata to the deptli of thirty, fifty, or even 

 seventy-five feet. This can be accounted for to a 

 large extent by climatic influences. The warm 

 waters, charged witli carbon dioxide, percolating 

 throughout the year the easily permeable strata, act 

 continuously as a chemical agent in the work of disin- 

 tegration; Avhile farther north not only the amoiint of 

 water, the temperature, and the chemical activity are 

 reduced, but for one-half of the year the soil is looked 

 up by frost from all access of decomposing agencies." 



The influence of these soils of disintegration upon 

 the agriculture of the regions in which they occur, 

 has often been noticed; and their bearing upon the 

 history of the use and manufacture of commercial 

 fertilizers in this country is no less clearly marked. 

 It would seem to be plain, that disintegration such as 

 this, when accompanied with or followed by denuda- 

 tion, would readily account for the accumulation, 

 and, so to say, concentration in 'pockets,' or other 

 places of rest, of any heavy or refractory minerals 

 which were originally contained, dispersed, in the 

 native rock; and that among the multitude of indi- 

 viduals thus thrown togetlier there would be much 

 greater likelihood of finding superior specimens than 

 can be obtained by searching tlie comparatively 

 meagre deposits that are formed at the north. 



The statement of Nordenskiold, above referred to, 

 is here given in condensed form. 



" Precious stones occur in Ceylon mainly in sand-beds, espe- 

 cially at places wbere streams of water have flowed which bave 

 rolled, crumbled down, and wabhed away a large part of the 

 softer constituents of the sand, so that a gravel baa been left 

 which contains more of the harder precious-stone layer than the 

 originally sandy strata or the rock from which they originated. 

 Where this natural washing ends, the gem collector begins. He 

 searches for a suitable valley, digs down a greater or less depth 

 from the surface to the Layer of clay mixed with coarse sand 

 resting on the rock, which experience has taught him to contain 

 gems. . . . The yield is very variable, sometimes abundant, some- 

 times very small. . . . Sapphires are found much more commonly 

 than rubies. . . . The precious stones occur in nearly every river 

 valley which runs from the mountain-heights in the interior of the 

 island down to the lowland. . . . But some one perhaps will ask, 

 Where is the mother-rock of all these treasures in the soil of Cey- 



